CVE-2020-9497: Apache Guacamole Information Disclosure

CVE-2020-9497 Overview

Apache Guacamole 1.1.0 and older versions contain an improper input validation vulnerability that fails to properly validate data received from RDP servers via static virtual channels. When a user connects to a malicious or compromised RDP server, specially-crafted Protocol Data Units (PDUs) can result in disclosure of sensitive information within the memory of the guacd process handling the connection.

Critical Impact

Attackers operating a malicious RDP server can exploit improper validation of static virtual channel data to leak memory contents from the guacd process, potentially exposing sensitive information from other user sessions.

Affected Products

• Apache Guacamole versions 1.1.0 and earlier
• Fedora 32 and 33 (packaged Guacamole versions)
• Debian Linux 9.0 (packaged Guacamole versions)

Discovery Timeline

• July 2, 2020 - CVE-2020-9497 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-9497

Vulnerability Analysis

This vulnerability exists in Apache Guacamole's handling of RDP static virtual channels, which are communication pathways used between RDP clients and servers for various functions like clipboard sharing, audio redirection, and device forwarding. The guacd daemon, which serves as the backend connection broker for Guacamole, processes incoming PDUs from RDP servers without adequate bounds checking or content validation.

When a user establishes an RDP connection through Guacamole to a malicious server, the attacker can send carefully crafted PDUs through static virtual channels. These malformed packets can trigger out-of-bounds memory reads within the guacd process, causing the daemon to inadvertently disclose portions of its process memory back to the attacker.

The information disclosure is particularly concerning in multi-tenant environments where a single guacd process may handle connections for multiple users. Memory leaked from the process could potentially contain session data, credentials, or other sensitive information from concurrent user sessions.

Root Cause

The root cause is improper input validation (CWE-20) in the guacd daemon's RDP protocol handler. The code responsible for processing static virtual channel PDUs does not adequately verify the length fields and boundaries of incoming data packets before reading or copying memory. This allows malformed PDUs with oversized or misaligned length values to cause the daemon to read beyond the intended buffer boundaries.

Attack Vector

The attack requires the victim to connect to a malicious or compromised RDP server through Apache Guacamole. The attack scenario involves:

1. An attacker sets up or compromises an RDP server accessible to Guacamole users
2. A victim user initiates an RDP connection through Guacamole to the attacker-controlled server
3. The malicious RDP server sends specially-crafted PDUs through static virtual channels
4. The guacd process improperly handles these PDUs due to insufficient validation
5. Memory contents from the guacd process are leaked back to the attacker

The exploitation requires local access in the sense that the attacker needs to control or compromise an RDP endpoint that users connect to. User interaction is required as victims must initiate the RDP connection to the malicious server.

Detection Methods for CVE-2020-9497

Indicators of Compromise

• Unusual RDP connection patterns to untrusted or newly added RDP servers
• Anomalous network traffic between guacd processes and RDP endpoints containing malformed PDU responses
• Unexpected memory access patterns or segmentation faults in guacd process logs
• Connections to RDP servers with suspicious or unverified SSL certificates

Detection Strategies

• Monitor guacd process logs for errors related to RDP virtual channel processing or memory access violations
• Implement network monitoring to detect RDP connections to unauthorized or suspicious external servers
• Deploy endpoint detection to identify unusual memory read patterns in the guacd daemon
• Audit Guacamole connection logs for connections to newly added or unauthorized RDP hosts

Monitoring Recommendations

• Enable verbose logging in Apache Guacamole to capture detailed connection and protocol information
• Set up alerting for guacd process crashes or unexpected terminations that may indicate exploitation attempts
• Monitor network traffic for anomalous RDP static virtual channel communication patterns
• Implement connection whitelisting to restrict RDP destinations to approved internal servers

How to Mitigate CVE-2020-9497

Immediate Actions Required

• Upgrade Apache Guacamole to version 1.2.0 or later immediately
• Restrict RDP connection destinations to trusted, internal servers only through Guacamole connection policies
• Implement network segmentation to limit guacd process exposure to untrusted RDP endpoints
• Review and audit existing Guacamole connection configurations for connections to external or untrusted RDP servers

Patch Information

Apache has addressed this vulnerability in Guacamole version 1.2.0. The fix implements proper validation of PDU length fields and boundary checking for data received through RDP static virtual channels. Users should upgrade to version 1.2.0 or later to receive the security patch.

For users on Fedora, updated packages are available through the official Fedora repositories. Debian Linux 9.0 users should refer to the Debian LTS Security Announcement for patched package availability.

Additional details are available in the Apache Guacamole Security Announcement and technical analysis from Check Point Research.

Workarounds

• Restrict Guacamole users to connecting only to pre-approved, trusted internal RDP servers
• Implement firewall rules to prevent guacd from initiating connections to external or untrusted networks
• Enable connection policy controls in Guacamole to limit which RDP hosts users can access
• Consider running guacd in an isolated container or sandbox environment to limit the impact of potential memory disclosure

# Restrict guacd to internal network connections only via iptables
iptables -A OUTPUT -m owner --uid-owner guacd -d 10.0.0.0/8 -p tcp --dport 3389 -j ACCEPT
iptables -A OUTPUT -m owner --uid-owner guacd -d 172.16.0.0/12 -p tcp --dport 3389 -j ACCEPT
iptables -A OUTPUT -m owner --uid-owner guacd -d 192.168.0.0/16 -p tcp --dport 3389 -j ACCEPT
iptables -A OUTPUT -m owner --uid-owner guacd -p tcp --dport 3389 -j DROP